Yes, there are a lot of rules governing the conduct of war. They cover such things as treatment of civilians and POWs, property crimes such as pillaging, deportations and population transfers, disallowed weapon types, and much more. You can read more [in this wiki article on the Geneva Convention](https://en.wikipedia.org/wiki/Fourth_Geneva_Convention#).
One thing to bear in mind is that there is no authority that is tasked with enforcing these rules. If a country is violating the laws of warfare, there is no police force that can bring them to justice. There are international organizations like the ICC which can try, but fundamentally the war must be won or lost first.
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